When a ground levelling operation is performed by a conventional tracked vehicle such as a bulldozer or the like, the vehicle is usually required to repeat forward/rearward movement several times. That is, while a blade is raised up to a desired height, it moves in the forward direction to push away soil, gravel or the like with the blade, thereafter, it once moves in the rearward direction, and subsequently, it moves in the forward direction again.
To easily perform ground levelling operation as mentioned above, many of the recently manufactured tracked vehicles have a function of automatically controlling the blade to a certain set height. Once an operator sets the blade height to a desired value by a blade height setting dial, the blade is automatically raised or lowered so as to allow the set blade height to be continuously maintained during the ground levelling operation.
When it is desired to change the set blade height in the course of the automatic blade height control, the operator operates a blade raising lever. The operation of the blade raising lever has priority over other operations.
In the conventional device of the foregoing type, however, when the blade height is manually changed in the course of the automatic blade height control by operating the blade raising lever, and thereafter, the blade height is subjected to automatic controlling again, the subsequent ground levelling operation is performed with the originally set blade height. As a result, in the conventional device, a stepped part appears in the region corresponding to the manual blade height control as shown in FIG. 3.
To perform a desired ground levelling operation as represented by a broken line in FIG. 3 having no stepped part, the conventional device requires such complicated operations that after shifting to the manual operation, the blade raising lever is continuously operated until the ground levelling operation is completed or the blade height setting dial is adjusted again after the completion of the manual operation.